Method and device for measuring multiple physiological properties of cells

ABSTRACT

A method of analyzing cells disposed in media within a vessel includes the steps of providing a vessel having an original volume of media about the cells, reducing the original volume of media about at least a portion of the cells to define a reduced volume of media, and analyzing a constituent related to the cells within the reduced volume of media. An apparatus for analyzing cells includes a stage adapted to receive a vessel holding cells and a volume of media, a plunger adapted to receive a barrier to create a reduced volume of media within the vessel including at least a portion of the cells, the barrier adapted for insertion into the vessel by relative movement of the stage and the plunger, and a sensor in sensing communication with the reduced volume of media, wherein the sensor is configured to analyze a constituent disposed within the reduced volume.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/502,417, filed on Sep. 10, 2003, the entire disclosure of whichis hereby incorporated by reference.

FIELD OF THE INVENTION

This application relates generally to high throughput screeningtechniques and, more specifically, to the measurement of theconstituents (analytes) of an extracellular medium surrounding livingcells. All of the patents, articles, and other references cited hereinform a part of this patent application and their respective disclosuresare incorporated herein by reference in their entirety.

BACKGROUND

Living cells typically consume nutrients and oxygen from the surroundingmedium, and return metabolic byproducts, including ions, carbon dioxide,lactate, and various proteins, to this extracellular environment. Therate of uptake and excretion of these analytes can provide valuableinformation regarding the metabolic processes underway inside the cells.

Conventional biological assays inherently exhibit significantlimitations. An ideal biological assay is homogeneous (i.e., does notrequire the introduction of a foreign agent such as a dye), non-invasive(i.e., has no deleterious effect on the biological process), and rapid.

Many tools have been developed to probe the mechanistic processes ofcells using internalized reporters such as fluorescent dyes. A devicethat is able to measure extracellular analytes using a non-invasive,homogeneous assay performed within a container that is compatible withexisting invasive tools would be particularly useful.

Some previous approaches relate to oxygen flux rate measurements, sincerespiration can be deemed to be a basic measure of cell viability. Manydevices have been developed to monitor respiration in vitro, throughdetermination of the rate of depletion of oxygen in the extracellularmedium. The earliest instruments relied on the change in total gaspressure in a sealed vessel, using the assumption that this change wasprimarily due to oxygen consumption.

In the 1960s, the Clark electrode (Clark, L. C. Jnr. Ann. NY Acad. Sci.1962; 102:29-45), and later the miniaturized Clark electrode, enabled amore specific measure of oxygen partial pressure. The relativecomplexity of the Clark design, and the fact that the electrode itselfconsumed oxygen, may have hindered its incorporation in a highlyparallel instrument suitable for widespread use. However, these deviceswere deemed successful enough to measure cell viability (Gesinski R M,Morrison J H, Toepfer J R. “Measurement of oxygen consumption of ratbone marrow cells by a polarographic method.” J Appl Physiol. 1968;24(6):751-754), to profile the toxic effects of drugs and chemicals(Shenoy M A, Biaglow J E, Varnes M E, Hetzel F W. “Inhibition ofcultured human tumor cell oxygen utilization by chlorpromazine.” Adv ExpMed Biol. 1983; 159:359-68), and to show the effect of agents such asinsulin on cellular metabolic processes (Panten U and Klein H. “O₂consumption by isolated pancreatic islets, as measured in aMicroincubation system with a Clark-type electrode.” Endocrinology 1982;111:1595-1600).

More recently, several oxygen sensors have been developed that canenable the design of a non-invasive, homogeneous readout of cellularrespiration. Fluorescent compounds, whose response is diminished by thephenomenon of oxygen-quenching, are now available. These compounds canbe embedded in an oxygen permeable membrane and exposed to cell media,and can be read using low cost, fiber coupled, semiconductor lightsources and sensors (Wolfbeis O S, 2002. “Fiber-Optic Chemical Sensorsand Biosensors.” Annal of Chem. 2002; 74:2663-2678).

An ion-sensitive field-effect transistor (ISFET), whose gate region canbe exposed to a liquid analyte, has been adapted to measure oxygenpressure using enzyme catalyzed conversion of oxygen (O₂) to H⁺ ionsthat can be detected by this sensor (Lehmann, M, Baumann W, BrischweinM, Gahle H-J, Freund I, Ehret R, Dreschler S, Palzer H, Kleintges M,Sieben U and Wolf B. “Simultaneous measurement of cellular respirationand acidification with a single CMOS ISFET. 2001.” Biosensors &Bioelectronics. 2001; 16:195-203).

Devices have been described and/or demonstrated that incorporateoxygen-quenched fluorophores, ISFETs and other oxygen sensors withinsample chambers containing bacteria or mammalian cells for the purposeof measuring respiration rate, viability, or the effect of drugs ortoxins. These devices range in size from fluorescent patches attached tothe interior wall of large cell culture bottles (Tolosa L, Kostov Y,Harms P, Rao G. “Noninvasive measurement of dissolved oxygen in shakeflasks.” Biotechnol Bioeng 2002 Dec. 5; 80(5):594-7), to fluorescentsensors embedded within microscopic flow cells fabricated usingmicrofluidics technology (Lähdesmäki I, Scampavia L D, Beeson C, andRuzicka J. “Detection of Oxygen Consumption of Cultured Adherent Cellsby Bead Injection Spectroscopy.” Anal. Chem. 1999; 71: 5248-5252), tomicrotitre plates with fluorescent compounds suspended within (O′RiordanT C, Buckley D., Ogurtsov V, O′Connor R., Papkovsky D B “A cellviability assay based on monitoring respiration by optical oxygensensor.” Anal. Biochem. 2000; 278(2):221-227) or deposited upon thewells (Woodnicka M, Guarino R D, Hemperly J J, Timmins M R, Stitt D,Pitner J B. “Novel fluorescent technology platform for high throughputcytotoxicity and proliferation assays.” Journal of BiomolecularScreening. 2000; 5:141-152).

Some patents describe a device for monitoring cells using anoxygen-quenched fluorescent compound that is placed in contact with abroth containing bacteria or mammalian cells. A fluorescence measurementof cells treated with a drug or toxin may be compared to a reference,purportedly to determine the effect of the compound on cellularrespiration. In an embodiment, cells are contained within a microplatethat is exposed to ambient air. Cells are maintained at a low density inorder to maintain viability in this configuration, because high celldensity would likely result in anoxia, acidification of the media, andcontact inhibition. Measurement times may, therefore, typically be tensof hours or days. In addition, the influx of ambient oxygen and lack ofcontrol of sample volume may allow only relative measurement to controlto be made. In another embodiment, to limit ambient oxygen influx,mineral oil is placed above the cell media. Because cell density istypically quite low, long measurement times are typically required.

A number of patents and publications describe oxygen flux measurementsystems incorporating small, closed sample chambers containing highdensities of cells. In these devices, an active perfusion system is usedto intermittently restore normal levels of dissolved oxygen, pH, andnutrients. None of these systems are designed or configured to enablethe user to easily culture cells, maintain their viability, runexperiments in parallel with high throughput, or run other types ofassays without detaching and moving the cells.

There have also been approaches to measuring cellular acidificationrate. Living cells produce protons (H⁺ ions) as a byproduct of variousmetabolic processes, including both aerobic and anaerobic respiration.Protons are also produced when ion exchange pumps on the surface ofeukaryotic cells are activated as a result of binding of a ligand with atransmembrane receptor or ion channel. In a fixed volume ofextracellular media, this proton flux causes a gradual acidificationthat can be measured using a pH sensor. Thus, an indication of metabolicrate and/or receptor activation can be determined from a precisemeasurement of extracellular acidification rate.

A number of pH sensors can be applied to the measurement of cell media.In addition to fluorescent and ISFET sensors similar to those describedpreviously, a light addressable potentiometric sensor has beenincorporated in an instrument for rapid measurement of proton flux(Parce W, Owicki J, Kercso K, Sigal G, Wada H, Muir V, Bousse L, Ross K,Sikic B, and McConnell H. 1989. “Detection of Cell-Affecting Agents witha Silicon Biosensor.” Science. 1989; 246(4927):243-247).

One patent describes a device employing a method for measurement ofextracellular acidification (pH) as an indicator of cellular metabolism.In this device, a small sample chamber containing a high density ofcells is intermittently perfused with media and closed to allowmeasurement of the pH change resulting from cellular proton excretion. Aseries of repetitive stop/flow cycles provides kinetic metabolic ratedata. Because the sample chamber, once assembled, is fixed in size andcontains a high density of cells, active perfusion is required toprevent cell death from the rapid acidification and depletion of oxygenfrom the media. The addition of a perfusion system to the device resultsin the need for relatively complex tubing, pumps, and other features,that create cleaning and sterilization problems for the user. Inaddition, when cells are to be treated with a drug using this device,the drug may need to be perfused over the cells for a relatively longperiod of time, thereby consuming large quantities of typically scarceand expensive compounds.

Other extracellular analytes can be measured using non-invasivetechniques. Carbon dioxide evolution can be determined from themeasurement of carbon dioxide (CO₂) partial pressure in the media usingvarious fluorescent sensors (Pattison R, Swamy J, Mendenhall B, Hwang C,and Frohlich B. “Measurement and Control of Dissolved Carbon Dioxide inMammalian Cell Culture Processes Using an in Situ Fiber Optic ChemicalSensor.” 2000. Biotechnology Prog. 16:769-774)(Ge X, Kostov Y, and GRao. High Stability non-invasive autoclavable naked optical CO₂ sensor.2003. Biosensor and Bioelectronics 18:pp. 857-865).

Other ions and chemical constituents can be measured using non-invasivetechniques based on optical or semiconductor sensors. In addition,larger molecules such as proteins can be measured using non-invasivetechniques that are sensitive to the binding of these molecules toantibodies that are attached to sensors exposed to the extracellularmedia (Flora K and J. Brennan. Comparison of Formats for the Developmentof Fiber-Optic Biosensors Utilizing Sol-Gel Derived Materials EntrappingFluorescently-Labeled Proteins. Analyst, 1999, 124, 1455-146).

Other physical phenomenon that support such sensors are surface plasmonresonance (Jordan & Corn, “Surface Plasmon Resonance ImagingMeasurements of Electrostatic Biopolymer Adsorption onto ChemicallyModified Gold Surfaces,” Anal. Chem., 69:1449-1456 (1997), gratingcouplers (Morhard et al., “Immobilization of antibodies in micropatternsfor cell detection by optical diffraction,” Sensors and Actuators B, 70,p. 232-242, 2000), ellipsometry (Jin et al., “A biosensor concept basedon imaging ellipsometry for visualization of biomolecular interactions,”Analytical Biochemistry, 232, p. 69-72, 1995), evanescent wave devices(Huber et al., “Direct optical immunosensing (sensitivity andselectivity),” Sensors and Actuators B, 6, p. 122-126, 1992),reflectometry (Brecht & Gauglitz, “Optical probes and transducers,”Biosensors and Bioelectronics, 10, p. 923-936, 1995) and Wood's anomaly(B. Cunningham, P. Li, B. Lin, J. Pepper, “Colorimetric resonantreflection as a direct biochemical assay technique,” Sensors andActuators B, Volume 81, p. 316-328, Jan. 5, 2002).

In general, the utility of devices incorporating these sensingtechnologies for the purpose of measuring secretion of proteins by cellsis limited by detection sensitivity. Sensitivity can be increased,typically by increasing cell density in the region proximal to thesensor surface. However, cellular health declines rapidly as celldensity increases, due to anoxia, acidification of the media, andcontact inhibition. It is possible, but generally undesirable, to adherecells directly to the sensor surface.

A need exists for the provision of a high cell density for measurementof analytes and a low density for maintenance of cell health and growth.While many devices have been developed for the purpose of measuring fluxrates of extracellular analytes, there exists a need to meetrequirements that may enable widespread use in the fields of biologicalresearch, drug discovery and clinical diagnostics. A need exists fordevices with high throughput and ease of use. A parallel configurationmay be desirable. Preferably, a tradeoff between long assay times andthe length of time to prepare the sample would be eliminated. Lack ofthese attributes may result in low sample throughput and thereforeincompatibility with modern drug discovery and diagnostic activities.

In addition, there is a need for an instrument that can be used tomeasure extracellular flux rates of cells in anon-invasive manner withina vessel that is commonly used for other high throughput assays, therebyallowing the use of the flux rate measurement as a quality control orcomplementary measurement to existing assays.

In summary, there is a need for a device that can meet the goals of dataquality, compatibility with existing experimental practices, andease-of-use, thereby enabling widespread adoption of a new technology.

SUMMARY

New methods and apparatus have been conceived and developed forproviding high cell density for measurement of analytes and low celldensities for maintenance of cell health and growth. The instantinvention can determine the flux rates of various extracellular analytesin minutes, can provide quantitative rather than relative readings, canbe used without adversely affecting the physiological state of the cellsunder test, and does not require an active perfusion or agitationsystem.

One feature of the invention is the temporary creation of asubstantially closed sample chamber within a vessel containing a lowdensity mixture of cells and media, and a sensor or plurality of sensorsfor measurement of analytes. Since a temporary sample chamber is createdwithin a larger vessel, media containing high levels of dissolved oxygenand other analytes, and normal pH, is supplied to the cells prior to,and immediately after a measurement is made. Using this feature, cellscan be grown, maintained for extended periods, treated with drugcompounds, and assayed using any of a variety of methods, while beingperiodically assayed for viability and respiration rate, withoutcompromising the cells.

Furthermore, the media containing cells need not be removed from thevessel; it is only displaced temporarily. Therefore, a minimal quantityof drug compound is required.

In addition, by precisely controlling the dimensions of the temporarysample chamber, a quantitative flux rate for extracellular analytes canbe determined easily. Therefore, an external reference is not required;a change in the flux rates of cells in a vessel can be determined frommultiple readings of this one vessel.

Elements of one embodiment of the invention include:

-   -   1. Temporary formation of a small, relatively impermeable sample        chamber (containing one or more cells, one or more sensors, and        a small amount of cell media) within a larger media-filled        vessel.

This configuration assists with:

-   -   increasing the rate of change of analytes in the media so that a        sensitive measurement can be made in a reasonably short time,        i.e., minutes vs. hours for some of the prior art methods;    -   eliminating the need for a reference, by overcoming the        following limitations of the prior art:    -   a. Low sensitivity (low cell density in the measurement broth        and therefore a small signal that may need to be measured);    -   b. Unknown sample volume (user variability in fill level of each        well and evaporation); and    -   c. O₂ influx from the surrounding environment (unless the entire        well is sealed with, e.g., a mineral oil coating as suggested by        the prior art, which results in a terminal experiment);    -   eliminating the need for complex fluidic systems to provide        intermittent perfusion to a flow cell, since a high ratio of        cells/media is only created temporarily in accordance with the        invention; and    -   development of a high sensitivity cell-based assay system for        other types of sensors, including SPR, SRU, etc., where the        analyte affected by the cells is affected at a low rate that is        difficult to measure;    -   2. The specific design of a device to accomplish the above,        including a stepped well and inverted, mushroom-shaped probe        with optical sensors on the bottom surface; and    -   3. Temporary insertion of the sensor described above into a        variety of vessels (including clear-bottom microplates)        containing cells.    -   This enables the use of substantially all conventional assays,        without the need to move cells or disturb their adhesion to the        vessel surface; and    -   Sensors can be Cleaned and reused in minutes.

It is one object of this invention to provide a rapid, non-invasive, andeasy-to-use method for determining various physiological properties ofliving cells. In particular, a device and method are described that canmeasure overall cellular metabolic and respirative rates, the relativeproportion of aerobic to anaerobic respiration, the relative rates ofconsumption of various metabolic substrates, the effect of stimulationof certain transmembrane and other cellular receptors, the rates ofproduction of various secreted factors, and cell viability.

The device and method can be applied in a variety of fields, includingbiological research, drug discovery, and clinical diagnostics. Thedevice can be used as a stand-alone instrument or in conjunction withexisting assay methods. For example, as a drug discovery tool, thedevice can be used to screen various compounds for an effect on cellularmetabolism, protein secretion, or intra/extra cellular ion exchange. Inaddition, the device can be used to replace more complex, invasive, andtime consuming methods for determining the toxic effects of compounds oncells or tissue samples. For this purpose, the device eliminates theneed for the addition of dyes and incubation of cells. The device canalso be used to determine the health of cells or tissue both before andafter a conventional assay is performed, thereby improving theperformance of such an assay.

In one aspect, the invention includes a method of analyzing cellsdisposed in media within a vessel. The method includes providing anoriginal volume of media about the cells, reducing the original volumeof media about at least a portion of the cells to define a reducedvolume of media, and analyzing a constituent related to the cells withinthe reduced volume of media.

One or more of the following features may be included. The reducedvolume of media about the cells may be increased to substantially theoriginal volume. A first concentration of the constituent may bedetermined, and a second concentration of the constituent may bedetermined at a predetermined time interval from the determination ofthe first concentration. A flux rate of the constituent may becalculated based on the first concentration and the secondconcentration.

The reduced volume may include, for example about 5-50% of the originalvolume, preferably about 5-20% of the original volume. In someembodiments, the reduced volume may be less than about 5% of theoriginal volume.

The cells may include bacteria, fungus, yeast, a prokaryotic cell, aeukaryotic cell, an animal cell, a human cell, and/or an immortal cell.At least a portion of the cells may be attached to a surface of thevessel. At least a portion of the cells may be suspended in the media.At least a portion of the cells may include living tissue.

The constituent being analyzed may include a dissolved gas (e.g., O₂,CO₂, NH₃), an ion (e.g., H⁺, Na⁺, K⁺, Ca⁺⁺), a protein (e.g., cytokines,insulin, chemokines, hormones, antibodies), a substrate (e.g., glucose,a fatty acid, an amino acid, glutamine, glycogen, pyruvate), a salt,and/or a mineral. The constituent may be extracted from the media by atleast a portion of the cells. The constituent may be secreted into themedia by at least a portion of the cells.

Analyzing the constituent may include sensing the presence and/or theconcentration of the constituent. Analyzing the constituent may includeby sensing a first concentration of a first constituent, sensing asecond concentration of a second constituent, and determining arelationship between the first concentration and the secondconcentration. Analyzing the constituent may include sensing a rate ofchange of a concentration of the constituent.

A sensor in contact with the media within the reduced volume may beused. The sensor may be a fluorescent sensor, a luminescent sensor, anISFET sensor, a surface plasmon resonance sensor, a sensor based on anoptical diffraction principle, a sensor based on a principle of Wood'sanomaly, an acoustic sensor, or a microwave sensor.

Analyzing the constituent may include determining a parameter such ascell viability, cell number, cell growth rate, response to at least oneof a drug, a toxin or a chemical, detection of an entity, andinternalization.

The method may include perfusing additional media through the vesseland/or replenishing the media in the vessel.

Reducing the volume of media may include disposing a barrier in thevessel, typically not causing displacement of the media out of thevessel. At least a portion of the barrier may include a sensor.Alternatively or additionally, the reduced volume of media may include asensor, such as a fluorophore. At least a portion of the vessel mayinclude a sensor.

The environment of at least a portion of the cells may be altered priorto reducing the original volume of media. The environment may be alteredby, e.g., exposing at least a portion of the cells to at least one of adrug, a chemical, or a toxin.

The environment of at least a portion of the cells may be altered afterreducing the original volume of media.

The method may include covering the vessel, sealing the vessel, and/orstirring at least a portion of the original volume of media in thevessel.

In another aspect, the invention features an apparatus for analyzingcells. The apparatus includes a stage adapted to receive a vesselholding cells and a volume of media; a plunger adapted to receive abarrier to create a reduced volume of media within the vessel includingat least a portion of the cells, the barrier adapted for insertion intothe vessel by relative movement of the stage and the plunger, and asensor in sensing communication with the reduced volume of media,wherein the sensor is configured to analyze a constituent disposedwithin the reduced volume.

One or more of the following features may be included. The sensor may beconfigured to analyze the constituent without disturbing the cells. Thevessel may include a well disposed in a microplate. The well may includea step. The barrier may be adapted to stir the media prior to analysisof the constituent.

The sensor may be, for example, a fluorescent sensor, a luminescentsensor, an ISFET sensor, a surface plasmon resonance sensor, a sensorbased on an optical diffraction principle, a sensor based on a principleof Wood's anomaly, an acoustic sensor, or a microwave sensor. At least aportion of the vessel may include the sensor, the reduced volume ofmedia may include the sensor, and/or at least a portion of the barriermay include the sensor.

The apparatus may include an automated electro-optical measurementsystem. The apparatus may also include a computer, with the automatedelectro-optical measurement system being in electrical communicationwith the computer.

The barrier may be biased relative to the plunger.

In another aspect, the invention features an apparatus for analyzingcells. The apparatus includes a vessel for holding cells and a volume ofcell media; a plunger adapted to receive a barrier to create a reducedvolume of media within the vessel including at least a portion of thecells, the barrier adapted for insertion into the vessel by relativemovement of the stage and the plunger without disturbing the cells, suchthat the reduced volume is less than about 50% of the volume of media;and a sensor in sensing communication with the reduced volume of media,wherein the sensor is configured to analyze a constituent disposedwithin the reduced volume.

In another aspect, the invention features a plate including multiplewells for holding media and cells. Each of at least a portion of thewells includes a seating surface for receiving a barrier a reducedvolume.

One or more of the following features may be included. A shape of theseating surface may be generally planar, arcuate, contoured, tapered,conical, stepped, or interlocking. The reduced volume within each of thewells may vary by less than about 10% of a mean volume of the wells,preferably by less than about 5% of the mean volume of the wells, morepreferably by less than about 1% of the mean volume of the wells. Theseating surfaces of the wells may each include a step disposed about aninner periphery of a respective well. The steps may lie in a step planedisposed above a bottom plane defined by bottoms of respective wells.The step plane and the bottom plane may be parallel planes. A height ofthe step plane may be less than about 1 millimeter (mm) above the bottomplane, preferably less than about 200 μm above the bottom plane, morepreferably less than about 50 μm above the bottom plane.

A fluorescent sensor may be disposed within at least one of the wells.At least one of the wells may include a transparent bottom. At least oneof the wells may include an opaque wall.

In another aspect, the invention features a barrier for analysis ofcells disposed in media in a vessel. The barrier includes a body portionfor insertion into the vessel, the body portion having a barrier surfacefor mating with a first surface of the vessel to create a reducedvolume.

One or more of the following features may be included. A shape of thebarrier surface may be generally planar, arcuate, contoured, tapered,conical, stepped, or interlocking. The barrier may include a cover formating with a second surface of the vessel.

A sensor may be disposed on the barrier surface for analyzing aconstituent of a media disposed about at least a portion of the cells.The sensor may include an optical sensor. The optical sensor may beadapted to sense a fluorophore.

A conductor may be coupled to the sensor and configured to conductsignals therefrom. The conductor may include an optical fiber and may bedisposed at least partially in the body portion. The barrier may includea readout for transmitting a signal from the sensor. The readout may bevisual, fiber, electronics on a post, and/or a plate reader from thebottom.

The barrier may include a plurality of barriers arranged to be receivedwithin a plurality of wells in a microplate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of one embodiment of thepresent invention, where the vessel is formed by a single well within amulti-well microplate, the cover and sensor assembly being shown in apre-measurement position;

FIG. 2 is a cross-sectional view of the cover and sensor assembly ofFIG. 1 in the measurement position;

FIG. 3 is a schematic illustration of a complete measurement system, inaccordance with one embodiment of the invention;

FIGS. 4 a and 4 b are schematic cross-sectional views of wells withdifferent seating surfaces;

FIGS. 5 a-5 c are schematic cross-sectional views of barriers thatinclude both a sensor assembly and a readout;

FIG. 6 is a graph showing the result of a study of the oxygenconsumption and extracellular acidification rates of typical mammaliancells, depicting the mean and standard deviation of a series of eightseparate measurements using one embodiment of the invention;

FIG. 7 is a graph showing the result of a study of the oxygenconsumption and carbon dioxide evolution rates of various numbers oftypical mammalian cells using one embodiment of the invention;

FIG. 8 is a graph showing the result of a study of the effect of thechemical compound 2,4, DNP on the rates of oxygen consumption, carbondioxide evolution, and extracellular acidification of typical mammaliancells using one embodiment of the invention;

FIG. 9 is a graph showing the result of a study of the effect of thechemical compound Rotenone on the rates of oxygen consumption andextracellular acidification of typical mammalian cells using oneembodiment of the invention;

FIG. 10 is a graph showing the result of a study of the effect of cellproliferation on oxygen consumption and extracellular acidificationusing one embodiment of the invention;

FIG. 11 is a graph showing the result of a study of the effect of thechemical compound Carbachol on the rate of extracellular acidificationof typical mammalian cells using one embodiment of the invention; and

FIG. 12 is a graph showing a comparison of the measured rates of oxygenconsumption of typical mammalian cells in a vessel with, and without,the formation of a small, enclosed sample chamber using one embodimentof the invention.

DETAILED DESCRIPTION

This invention enables the temporary creation of a highly concentratedvolume of cells within a larger volume of cell media, in order to allowsensitive measurements of the change in constituents of the media thatresult from biological activity of the cells. By temporarily, ratherthan permanently, reducing the media volume (and therefore concentratingthe cell/media mixture), cells are exposed to a non-normal environmentfor only a brief period of time and are therefore not adversely affectedby the measurement process.

In one embodiment of the invention, cells are grown or placed on thebottom of a vessel containing sufficient type and volume of media tosupport growth for an extended period of time. A sample chamber isformed in the bottom of the vessel, consisting of the bottom of thevessel and vertical walls, such that the enclosed volume is sufficientto contain the cells plus a reduced volume of media.

A barrier, having a diameter slightly less than the inside diameter ofthe vessel, is located above the sample chamber on a movable actuator.Upon actuation, the barrier may be raised above the level of liquid inthe vessel, or lowered into the liquid and on to the vessel walls,forming a sample chamber that is relatively impervious to the diffusionof analytes from the sample chamber to and from the bulk media now abovethe cover.

A cross-sectional view of a representative embodiment is shown inFIG. 1. The drawing details a vessel 100 that is typical of one well 110within a multi-well microplate. The walls of this single well 110 formthe vessel 100 that contains live cells 120 and cell growth media 130.Cells may or may not adhere to a bottom surface 132 of the vessel, andthe bottom surface may be treated or coated to encourage adherence.Alternatively, cells may be suspended within the media and may be forcedto the bottom of the vessel using gravity or centrifugal force.

A barrier 140, having a diameter slightly less than an inside diameterof the vessel 100, is used to form a cover that defines a sample chamber150 within the vessel. Barrier 140 may have a diameter d₁ of, e.g., 6mm, and vessel 100 may have an inside diameter d₂ of, e.g., 7 mm. InFIG. 1, the barrier 140 is shown in a pre-measurement position withinthe vessel. To effect a measurement, a manual or motorized plunger(actuator) can then be used to reposition the barrier 140 slightly abovethe bottom surface 132 of the vessel 100 as shown in FIG. 2 by loweringthe barrier 140 or raising the well 110. Orienting the bather to theposition shown in FIG. 2 prior to measurement defines the sample chamber150 having a reduced volume of media, thereby enhancing measurementsensitivity.

A single vessel of nearly any size may be fabricated, or multiplevessels may be fabricated in a one- or two-dimensional arrangement. Inone embodiment, a two-dimensional pattern of vessels corresponding tothe pattern and dimensions of a microplate, as described by the Societyfor Biomolecular Screening standards for microplates (“SBS-1 Footprints”and “SBS-4 Well Positions,” both full proposed standards updated May 20,2003), and containing a total of 12, 24, 96, 384, 1536, or any othernumber of individual wells may be fabricated.

The vessel and sample chamber may typically be formed using plasticmaterial such as, for example, polystyrene or polypropylene, with thebottom clear and the sides colored black to reduce optical cross-talkfrom one well to another.

A variety of types of barriers may be employed to temporarily reduce thevolume of media about the cells without causing displacement of mediaout of the vessel, such as a simple planar cover lowered vertically, asliding cover extended horizontally, or a pair of disks with cutoutsthat can be rotated to act as a valve. It is desirable that the barriernot disturb, i.e., not move, the cells or the media proximal to thecells, in order to reduce the required settling time prior to ameasurement.

A complete measurement system can be assembled using the componentsshown in FIG. 3. A vessel 300, e.g., a plate such as a microplateincluding a plurality of wells 302, is placed upon a translation stage310. The microplate is disposed beneath an array of barriers 320disposed on a plunger 322 adapted to receive the barriers and an arrayof pipettors 330. Each of at least a portion of the wells includes aseating surface (see, e.g., FIGS. 4 a and 4 b) adapted to receive one ofthe barriers. Barriers 320 may include sensors. An original volume ofmedia may be disposed in the wells. Using manual or motorized actuation,the barriers and pipettors may be lowered into the microplate wells tocreate a reduced volume of media within the wells. The reduced volumemay be less than, e.g., 50%, of the original volume of media. Thebarriers are adapted for insertion into the vessel, i.e., into thewells, by relative movement of the stage 310 and the plunger 322. Thebarriers and pipettors may also be lowered into one of several fluidreservoirs 340 containing wash buffers and calibrants. When the barrierscreate the reduced volume of media within the vessels, sensors may be insensing communication with the reduced volume of media and may beconfigured to analyze one or more constituents disposed within thereduced volume. The sensors may be interrogated by an optical interfaceconsisting of illumination sources (e.g., light emitting diodes) andlight detectors (e.g., photodiodes), with appropriate band-limitingfilters interspersed between the optical elements. A computer andsoftware 350 perform actuation, calibration and measurement functions.

A change in the temperature of the media within the sample chamber mayresult in unwanted measurement errors from at least two sources. First,the capacity of the media to hold dissolved gasses changes withtemperature, and therefore a change in temperature may cause an apparentchange in dissolved gas concentration as the media seeks equilibriumwith the surrounding environment. Second, the measurement properties ofmany types of sensors changes with temperature.

To ensure accurate and repeatable measurements, the temperature of areduced volume of media in the vessel may be controlled or a correctionfactor may be applied to the measurement. Because evaporation inducescooling of the liquid media, control of evaporation may be desired toreduce thermal drift, thermal gradients, and gas exchange.

Providing environmental and temperature control for the sample chambermay reduce unwanted impact on the measurement process. For example,uncontrolled temperature changes of the media surrounding the cells candirectly impact the rate of apparent oxygen consumption. Oxygen willnaturally off-gas from media as it warms, thus introducing theappearance of a change in cellular respiration when, in fact, the ratechange observed is a natural function of dissolved gas seekingequilibrium as the temperature increases. Similarly, any evaporationfrom the media due to other uncontrolled environmental conditions suchas humidity or exposure to air currents can artificially impact themeasurements made from various sensors including those of dissolvedgases, ions, and temperature.

Using this measurement system, an assay cycle is initiated by mating thesensors/barriers with the vessel walls to form closed sample chamberswith reduced volume of media containing the cells. The rate and patternof actuation of the barriers may be programmed to prevent rapid motionof the media that may disturb the cells, i.e., displace the cells by orcause shear stress on the cells, and may be alternated to provide fluidmotion for stirring of the media, as desired.

Additionally, the barriers may be independently biased, for example, byusing springs or other force elements, to ensure adequate seating of thecovers in all of the wells, simultaneously.

The electro-optical interface and computer are then used to measure thechange in response of the sensor or sensors resulting from the change inconcentration of extracellular analytes. The rates of consumption orproduction of analytes may be determined by making multiple readingsover a period of minutes and then calculating the slope between selectedmeasurement points. Once the measurement sequence is completed, thesensor/covers are retracted to expose the cells to the full volume ofmedia within each vessel.

The measurement system may include provisions for single ormultiple-point calibration of the analyte sensors. For example, tworeservoirs containing liquid of known, but different pH, oxygen, CO₂, orother analyte levels may be incorporated, and a two-point (gain andoffset) calibration may be performed periodically. Alternatively,“factory” pre-calibration of the sensors may be used to eliminate theneed for field calibration, or to reduce the calibration to a singlepoint (offset) correction.

Referring to FIG. 4 a, in one embodiment, a microplate is used toprovide a plurality of measurement vessels in a standardized pattern. Byincorporating a seating surface 400 in each well, a precise reducedvolume of media can be maintained about the cells during the measurementperiod. The reduced volume within each of the wells disposed in a platemay vary by less than about 10% of a mean volume of the wells. In someembodiments, the reduced volume may vary by less than 5% of the meanvolume of the wells, and in some embodiments, the reduced volume mayvary by less than 1% of the mean volume of the wells. The seatingsurface 400 or steps may lie in a step plane 410 disposed above a bottomplane 420 defined by bottoms 430 of respective wells, with the stepplane 410 and the bottom plane 420 being parallel planes. The height ofthe step plane is generally less than about 1 mm above the bottom planeand typically less than 50 μm to 200 μm above the bottom plane.

Referring to FIG. 4 b, in another embodiment, a sloped surface 435 isincorporated to prevent the adhesion of cells on the seating surface440. Any of a variety of alternative mating cover and seating surfacescan be employed, in various combinations and permutations, includingthose that are generally planar, arcuate, contoured, tapered, conical,stepped, interlocking, etc. What is generally desired is that matingfeatures reliably and repeatably isolate the reduced volume from theoriginal volume, such that the reduced volume has a generallypredetermined or known capacity. Auxiliary seating components, such asO-rings, or resilient or compliant sealing lips, flaps, or otherfeatures may be employed on the covers or in the wells to enhance theseal, as desired.

The barrier can be fabricated to include a sensor assembly and a readoutfor transmitting a signal from the sensor assembly. FIG. 5 a shows across-sectional view of a barrier formed from the combination of atubular solid support 500 and a removable cover 510 or sheath, having anenlarged distal end that forms a structure upon which one or moreoptically-coupled sensors 520 are attached. In one embodiment, thesheath may be fabricated from a material that is either disposable orsterilizable in order to prevent contamination of the cell media. Areadout 530 may be in the form of optical fibers disposed within thetubular support for communication between the sensors and anelectro-optical measurement system 540. The electro-optical measurementsystem 540 may incorporate a source of illumination, an opticaldetector, spectral filters, and signal processing components. Theelectro-optical measurement system 540 may be automated. In someembodiments, the electro-optical measurement system 540 may be inelectrical communication with computer 350 (see FIG. 3).

FIG. 5 b shows an alternative arrangement in which the sensors areilluminated by an external light source 550. Electro-optical measurementsystem 540 may include separate components, i.e., an optical measurementsystem 560 and an illumination system 570. The optical measurementsystem 560 and illumination system 570 may be automated. In someembodiments, the optical measurement and illumination systems 560, 570may be in electrical communication with computer 350. Referring to FIG.5 c, in an alternative embodiment, the electro-optical measurementsystem 540 includes optical and measurement components 580 locatedwithin the tubular support 500 and an external electronic measurementsystem 585. The optical and measurement components may communicate withthe external electronic measurement system 585 through a cable 590.

Any form of signal communication can be employed, as desired. Such formsof signal communication might include simple visual interrogation of asignal change such as a change in color; fiber optic signalcommunication coming from any side of the vessel; a laser or CCD-basedplate reader interrogating the signal from the bottom of a transparentvessel.

In practice, many different configurations of vessels, barriers, andsensors may be employed. The total vessel volume may range from manyliters to a fraction of a microliter (ml), but is generally less thanabout 1 ml. The ratio of the reduced volume of media enclosed within thetemporary sample chamber to an original volume of media provided in thevessel may range from about 50% to less than about 5% and even as low asless than about 1%, but is typically in the range of 5-20%.

Many different types and numbers of cells can be analyzed, includingbacteria, fungus, yeast, prokaryotic and eukaryotic cells, animal orhuman cells, etc. Cells may adhere to the vessel wall or may besuspended within the media. Immortalized cells, native and primarycells, and homogenized or sliced tissue may be analyzed. A centrifugemay be used to concentrate cells within the sample chamber region of thevessel.

Any number of constituents of the media may be analyzed, includingdissolved gasses, ions, proteins, metabolic substrates, salts andminerals. These constituents may be consumed by the cells (such as O₂),or may be produced by the cells either as a byproduct (such as CO₂ andNH₃) or as a secreted factor (such as insulin, cytokines, chemokines,hormones or antibodies). Ions such as H⁺, Na⁺, K⁺, and Ca⁺⁺ secreted orextracted by cells in various cellular metabolism processes may also beanalyzed. Substrates either consumed or produced by cells such asglucose, fatty acid, amino acids, glutamine, glycogen and pyruvate maybe analyzed. Specialized media may be used to improve the sensitivity ofthe measurement. For example, the change in pH resulting fromextracellular acidification can be increased by using a media withreduced buffer capacity, such as bicarbonate-free media.

The analysis performed using this method may simply detect the presenceof a constituent in the media, or may quantitatively analyze the amountand change in concentration, volume, or partial pressure of aconstituent. With the incorporation of multiple sensors, one or moreratios of constituents may be analyzed. As an example, the ratio ofanaerobic to aerobic respiration utilized by the cell can be determinedfrom a calculation of the ratio of oxygen consumption to extracellularacidification rate that is enabled by a measurement of changes in oxygenpartial pressure and pH of the extracellular media. Analysis may includesensing a first concentration of a first constituent, sensing a secondconcentration of a second constituent, and determining a relationshipbetween the first concentration and the second concentration.

The type of sensors utilized include oxygen sensors, such asoxygen-quenched fluorescent sensors, enzyme-coupled ISFET sensors,miniature Clark electrodes, or other oxygen sensors; pH sensors,including fluorescent sensors, ISFET sensors, pH sensitive dye sensors,LAP sensors, or other pH sensors; CO₂ sensors, including bicarbonatebuffer coupled and ammonium dye coupled fluorescent sensors as well asother CO₂ sensors; various ion and small molecule sensors; largemolecule sensors including surface plasmon resonance sensors and sensorsexploiting the principle of Wood's anomaly; acoustic sensors; andmicrowave sensors.

The method may be used to measure any number of attributes of cells andcellular function. For example, cell viability and metabolic rate may bedetermined from measurements of oxygen consumption rate, extracellularacidification rate, or other metabolic analyte fluxes. By comparison ofone or more analyte flux rates to a known rate per cell, cell number maybe determined and therefore growth rates can be monitored.

The number of sensors used may range from one to many hundreds. Sensorsfor dissolved gasses may be placed within the sample chamber, but not indirect contact with the media. Other sensors, however, should be indirect contact with the media and in close proximity to the cells. Thismay be accomplished by mixing an indicator compound, e.g., afluorophore, with the cell media, or by embedding the indicator in acompound that is permeable to the analyte to be measured. The embeddedindicator may then be attached to any surface of the sample chamberregion of the vessel.

In one embodiment, one or more sensors may be attached to the lowersurface of the barrier, so as to be exposed to the extracellular mediaupon lowering of the barrier. One example of a sensor for this purposeis a fluorescent indicator, such as an oxygen-quenched fluorophore,embedded in an oxygen permeable substance, such as silicone rubber.

Sequential measurements of a single group of cells may be made atpredetermined time intervals to analyze the effect of changes in theextracellular environment on their function, for example to examine theeffect of exposure to a drug, chemical, or toxin. In this method, thevolume of media surrounding the cells is first reduced, the constituentsof the media are measured, and the volume is restored to its originalvalue, as previously described. The environment surrounding the cells isthen altered, such as by adding a chemical that activates atransmembrane receptor, changing the dissolved oxygen level, or adding anutrient. One or more additional measurement cycles are then performedusing the temporarily reduced volume method, to analyze the effect ofthe altered extracellular environment.

At any time during the sequence of measurements, the cell media may bereplenished. In this way, sequential measurements can be made over aperiod of minutes, hours, or days. Any one of several differentapproaches may be followed to replenish the media. Media may bereplenished by substantially removing part or all of the media withinthe full volume of the vessel using standard manual or automatedpipetting instruments. Alternatively, media may be replenished onlywithin the reduced volume of the vessel when a barrier is lowered intoposition. In the latter method, media may be replenished by fluidicextraction and delivery from a top side of the vessel through a portalin a plunger mechanism or through a portal built into any one of thesides or bottom of the vessel.

The introduction of an environment altering constituent such as achemical, dissolved gas or nutrient may also be applied either to thefull volume of the vessel as noted above or alternatively to only thereduced volume of the vessel. In the latter embodiment, the volume ofmedia surrounding the cells is first reduced, the constituents of themedia are measured, and the volume is restored to its original value, aspreviously described. The volume is then again reduced and theenvironment immediately surrounding the cells within only the reducedvolume is then altered, by the addition of a constituent through aportal in the plunger or elsewhere in the vessel defining the reducedvolume. One or more measurements are made in the presence of theconstituent. After this measurement cycle, the media within the reducedvolume may be exchanged one or more times to flush out the constituentbefore exposing the cells once again to the full original volume. Thisapproach may provide a benefit of reducing the volume of compoundrequired. It may also provide the possibility of studying isolatedeffects without contaminating the entire volume, thereby, in effect,simulating a flow system in wellplate format.

EXAMPLES

The following examples illustrate certain exemplary and preferredembodiments and applications of the instant invention, but are notintended to be illustrative of all embodiments and applications.

Example 1 Repetitive Measurement of the Basal Respiration andAcidification Rates of C2C12 Myoblasts

A prototype device was fabricated in order to evaluate variousproperties and potential applications of the invention.

The device included a cylindrical vessel, fabricated from polycarbonatematerial, and designed to receive a 12 mm diameter, polycarbonatemembrane assembly (Corning Snapwell™ P/N 3802) with a pore size ofapproximately 3 μm. A cylindrical polycarbonate cover could betemporarily inserted into the vessel to form a smaller sample chamber,approximately 1.5 mm high, with a volume of about 160 microliter (μl). Aseries of bores around the perimeter of the vessel allowed the insertionof three 500 μm diameter optical fibers. The distal tip of each opticalfiber was coated with a fluorescent sensing material to form abiosensor.

The three biosensors were designed to measure the partial pressures ofO₂ and CO₂, and the pH of the media contained within the vessel. Onefiber tip was coated with a matrix of Ruthenium dyes, encapsulated inoxygen permeable silicone rubber, to provide a readout of dissolvedoxygen concentration. A second fiber tip was coated with a complex ofFluoroscene dye encapsulated in silicone rubber, to provide a readout ofH⁺ ion concentration (pH). A third biosensor was fabricated by using aCO₂ permeable membrane to create a small reservoir of NaHCO₃ surroundinga HydroxyPyrene Trisodium Salt (HPTS) pH sensitive dye. A change in CO₂concentration in the cell media would then cause a change in pH of thisencapsulated reagent resulting in a measurable change in the fluorescentproperties of the pH sensitive dye, and this change was calibrated toprovide quantitative CO₂ concentration data.

Light emitting diodes were used to illuminate the three optical sensorsat various wavelengths as shown in Table 1, in terms of nanometers (nm).Also shown in Table 1 are the wavelengths used to sense the fluorescentemission of each sensor. In each case, both analyte sensitive(“sensor”), and analyte insensitive (“reference”) fluorescent propertiesof the dyes were measured to minimize unwanted drift and interference.Dichroic splitters were used to couple individual fiber/dye assembliesto a pair of photodiodes/filter sets (O₂ sensor) or a pair of LED/filtersets (pH and CO₂).

TABLE 1 Analyte sensor excitation and emission wavelengths SensorReference Sensor Reference Excitation Excitation Emission EmissionOxygen 488 nm 488 nm 610 nm 535 nm PH 464 nm 435 nm 530 nm 530 nm CO₂460 nm 415 nm 530 nm 530 nm

Each sensor was calibrated once using multiple measurement points and apolynomial regression method to establish a nonlinear calibration curve.

Sensors were then recalibrated daily using a two-point calibrationmethod. pH sensors were calibrated by sampling the optical responsewhile submerged in a buffer solution with pH of 6.0 for 2 minutes, thenin a solution with pH 8, each for two minutes. Oxygen and CO₂ sensorswere calibrated using data points acquired while both sensors weresubmerged for two minutes in a saline bath purged with room air,followed by a bath purged with 10% CO₂/90% N₂.

During a typical assay, approximately 1.5×10⁵ cells were placed in thevessel along with 500 μl of liquid media, resulting in a cell density of3×10⁵ cells/ml. To perform a measurement, the cylindrical cover wastemporarily inserted into the vessel. The cover displaced liquid media,but not cells, to form a smaller sample chamber with a volume of 160 μland a therefore a cell density of approximately 1×10⁶ cells/ml. Thisresulted in more than a 6× increase in the rate of change of analyteswithin the media in proximity to the biosensors.

In order to evaluate the ability of the prototype device to reproduciblymeasure extracellular analytes flux rates, 1.5×10⁵ undifferentiatedC2C12 murine skeletal muscle cells (obtained from ATCC, Manassas, Va.)were seeded on each of eight separate 12 mm diameter polycarbonatemembranes which were then incubated at 37° C. for a period of 12 hours.

In a sequence of tests, wells were removed from the incubator, inspectedvisually, and placed into the measurement device. 160 μl of bicarbonate(NaHCO₃)-free DMEM Medium (obtained from Specialty Media, Phillipsburg,N.J.) was then added, and the device was assembled to form an enclosedsample chamber. The concentration of each analyte (partial pressures ofO₂ and CO₂, and pH as an indicator of proton concentration) was thenmeasured every 8 seconds for a period of 20 minutes, and the averagerate of change of each analyte was calculated over a four minute periodfrom t=12 minutes to t=16 minutes.

To determine the extracellular flux rates of O₂ and CO₂, the rates ofchange of partial pressures were divided by volume of each analytesavailable in the media (moles) to result in a value expressed innmol/minute. The rate of acidification was expressed in mpH units/min(multiplied by 20 for scaling on the chart), but can easily be shown asprotons per minute by calculating the number of available electrons inthe media buffer within the known sample volume.

The mean and standard deviation of the dissolved oxygen and pH decayrates for the series of eight tests are shown in FIG. 6. As shown, theseflux rates are highly reproducible in the prototype device.

Example 2 Measurement of Basal Respiration and Acidification Rates forVarious Cell Densities

The experimental device described in Example 1 was used to investigatethe relationship between cell number and oxygen and CO₂ flux rates.Varying numbers (1.0×10⁵−4.0×10⁵) of C2C12 myoblasts were seeded on 12mm diameter polycarbonate membranes (Corning Snapwell™) which were thenincubated at 37° C. for a period of 12 hours.

Wells were then removed from the incubator, inspected visually, andplaced into the measurement device. 150 μl of NaHCO₃-free DMEM Medium(obtained from Specialty Media, Phillipsburg, N.J.) was then added, andthe device was assembled to form an enclosed sample chamber. Theconcentration of each analyte was then measured every 5 seconds for aperiod of 20 minutes, and the average rate of change from t=10 minutesto t=20 minutes from start was computed. The resulting flux rates areshown in Table 2 and in graphical form in FIG. 7.

TABLE 2 Measuring Metabolic Analytes from Varying Titrations of C2C12Myoblasts Cell # O₂ CO₂ pH (000) Rate Rate Rate CO₂/O₂ O₂/pH 400 0.771.22 0.023 0.88 0.33 300 1.06 1.22 0.021 1.15 0.50 200 0.70 1.08 0.0190.93 0.35 150 0.57 0.90 0.021 1.63 0.26 100 0.29 0.68 0.013 2.36 0.22

The data in Table 2 shows, as expected, that increasing cell densityincreases analyte flux rates in a near-linear fashion for most celldensities. Above a density of 3×10⁵ cells, oxygen flux did not increaseas rapidly, presumably due to contact inhibition and crowding effects.

The device can therefore be used to evaluate the effect of high celldensities on metabolic rates.

Example 3 The Effect of 2,4 DNP on C2C12 Myoblasts

The chemical compound 2,4 DNP can be used to uncouple mitochondrialrespiration from ATP synthesis by disassociating the linkage between therespiratory chain and the phosphorylation system. In the presence ofthis compound, it is known that oxygen consumption will increasedramatically, while proton flux remains relatively constant.

In this experiment, C2C12 myoblasts were seeded on 12 mm diameterpolycarbonate membranes and incubated for 12 hours. Wells were thenremoved from the incubator, inspected visually, and placed into themeasurement device. 160 μl of NaHCO₃-free DMEM medium was then added,and the device was assembled to form an enclosed sample chamber.

The dissolved concentrations of O₂ and CO₂, and the pH in the media werethen measured every 5 seconds for a period of 20 minutes in order todetermine a control baseline for each analyte flux. Once the baselinewas established, a sequence of experiments were performed where varyingdoses of 2,4 DNP (obtained from Sigma, St. Louis Mo.) were added to thecell media and a 20 minute measurement of analyte flux rates wasperformed. A control experiment was also performed using the highestdose of 2,4 DNP, but without cells. The data from the dose response isshown in. Table 3 and FIG. 8.

TABLE 3 Effect of 2,4DNP on C2C12 Myoblasts 2,4 DNP Dose O₂ Rate CO₂Rate pH Rate (μM) (nM/min) (nM/min) (pH/min) CO₂/O₂ O₂/pH  0 0.38 2.220.024 5.83 0.16  10 1.26 2.78 0.028 2.18 0.46  50 1.99 4.24 0.031 2.130.64 100 2.30 4.59 0.032 2.00 0.73 100 −0.18 0.15 0.001 −0.84 −1.30 NoCellsThe data in Table 3 shows that as predicted, treatment with 2,4 DNPcauses a dose-dependent increase in O₂ consumption rates while havinglittle effect on extracellular acidification.

Example 4 The Effect of Rotenone on C2C12 Myoblasts

Rotenone is known to inhibit cellular respiration by blocking NADHdehydrogenase in the respiratory chain. C2C12 Myoblasts were used toshow this effect. 1.5×10⁵ C2C12 myoblasts were seeded on membranes,incubated and placed in the measurement system along with 150 μlNaHCO₃-free DMEM medium of as described in Example 3.

The dissolved concentrations of O₂ and CO₂, and the pH in the media werethen measured every 5 seconds for a period of 20 minutes in order todetermine a control baseline for each analyte flux. Once the baselinewas established, a sequence of experiments were performed where varyingdoses of Rotenone (obtained from Sigma, St. Louis Mo.) were added to thecell media and a 20 minute measurement of analyte flux rates wasperformed. A control experiment was also performed using the highestdose of Rotenone, but without cells. The data from the dose response isshown in Table 4 and FIG. 9.

TABLE 4 Effect of Rotenone on C2C12 Myoblasts Rotenone O₂ CO₂ pH DoseRate Rate Rate CO₂/O₂ O₂/pH  0 0.58 1.50 0.027 2.56 0.22  25 0.44 1.760.026 4.00 0.17  50 0.26 2.08 0.031 8.02 0.08 100 0.19 2.34 0.023 12.180.09 200 0.04 2.24 0.027 69.84 0.02 200-No −0.07 0.14 0.001 7.51 −0.51CellsThe data in Table 4 demonstrates that, as expected, treatment withRotenone causes a dose-dependent decrease in O₂ consumption rate inthese cells.

Example 5 Measurement of Respiration and Acidification Rate ChangesResulting from Cell Proliferation

The experimental device described in Example 1 was used to investigatethe relationship between cell proliferation and oxygen, CO₂ and protonflux rates. 5.0×10⁴ C2C12 myoblasts were seeded on 12 mm diameterpolycarbonate membranes and then incubated at 37° C. 12 hours afterbeing seeded, cells were placed in DMEM serum-free media (Gibco,Carlsbad, Calif.) to inhibit proliferation. After 24 hours, half of thecells were switched to DMEM serum-containing media to stimulateproliferation, while the other half were maintained in serum-free media.

Wells were then removed from the incubator, inspected visually, andplaced into the measurement device. 57 μl of NaHCO₃-free DMEM Medium(obtained from Specialty Media, Phillipsburg, N.J.) was then added andthe device was assembled to form an enclosed sample chamber. Theconcentration of each analytes was then measured every 8 seconds for aperiod of 20 minutes, and the average rate of decay from t=10 minutes tot=20 minutes from start was computed. The resulting flux rates are shownin Table 5 and in graphical form in FIG. 10.

TABLE 5 Effect of cell proliferation on extracellular analyte fluxes O2Rate PH Rate * 10 (nMoles/min) (PHU/min) 50 K Stimulated 0.311 +/− 0.0910.188 +/− 0.020 50 K Starved 0.082 +/− 0.019 0.102 +/− 0.050The data in Table 5 demonstrates that, as expected, cell proliferationresults in an increase in oxygen consumption and the rate ofextracellular acidification.

Example 6 Measurement of G-Protein Coupled Receptor Activation in CHO-M3Cells

Previous studies have shown that stimulation of transmembrane receptorsoften causes a rapid increase in extracellular acidification rate,resulting primarily from acute activation of ion exchange pumps. In thisexperiment, the prototype device was used to detect a change inextracellular acidification rate following treatment of cells with areceptor agonist.

Chinese hamster ovary (CHO) cells were transfected to over-express themuscarinic receptor subtype m3. The prototype device described inExample 1 was then used to monitor O₂ consumption, CO₂ production, andextracellular acidification rates, following treatment with thewell-known, general acetylcholine receptor agonist, Carbachol.

Materials and Methods: Cell culture reagents were obtained from GibcoBRL (Grand Island, N.Y.). Carbachol was purchased from Sigma ChemicalCo. (St. Louis, Mo.). Bicarbonate-free DMEM medium was obtained fromSpecialty. Media (Phillipsburg, N.J.). Polycarbonate membrane snapwells(12 mm diameter, 3 μm pore size) were obtained from Corning (Corning,N.Y.). CHO cells expressing m3-muscarinic receptors (CHO-M3 cells) wereobtained from the American Type Tissue Culture (ATCC; Manassas, Va.).Cells were cultured in Ham's F-12 medium supplemented with 10% fetalbovine serum (Hyclone), 1% GlutaMax and 0.1% Gentamicin and weremaintained in a 5% CO₂ incubator. Cells were subcultured when theyreached 80% confluency. CHO-M3 cells were seeded at a density of 2×10⁵onto a snapwell 24 hours prior to use. Immediately prior to testing,cells on snapwells were switched to bicarbonate-free DMEM mediumcombined with 3.7 g/l NaCl to maintain osmolarity (medium pH 7.4-7.5).

Protocol Description: Probes were calibrated immediately prior totesting. The bottom of the test vessel was filled with bicarbonate-freemedium. The snapwell was removed from a 5% CO2 incubator, and theregular growth medium (Ham's F-12) was replaced with bicarbonate-freeDMEM medium. Thereafter, the snapwell was placed into the test vessel.Bicarbonate-free medium was pipetted onto the top of the snapwell, andthe cover piece of the test vessel was placed gently on top of thesnapwell and screwed into place, compressing the assembly. The probesoftware was started, and the pH, CO₂ and O₂ analytes were measured overthe next 3.5 hours. Following the initial 1.5 hours of perfusion at arate of 78 μl/min, a series of stop flow (10 minutes each) and mediumre-perfusion (10 minutes each, 78 μl/min) cycles were started. Duringthe last 2 minutes of medium re-perfusion cycle number 5, 100 μMcarbachol was perfused across the snapwell. During re-perfusion number6, bicarbonate-free DMEM medium was once again perfused across thesnapwell. A rate of change for the analytes was calculated during eachstop flow cycle.

Results:

CHO-M3 Baseline followed by 100 μM Carbachol Treatment

The first four series of perfusion/stop flow cycles were done toestablish a noise band on the three analytes prior to carbacholtreatment during perfusion number 5. Bicarbonate-free mediumre-perfusion during per perfusion number 6 and the rates calculatedduring stop flow number 6 were to assess potential continuingpost-carbachol treatment effects on analytes' rates. Resulting data areshown below in Table 6 and in FIG. 11.

TABLE 6 Effect of carbachol exposure on oxygen consumption, carbondioxide evolution, and extracellular evolution Rate O₂ (nMol/ CO₂ pHO₂/CO₂ O2/pH Summary min) (nMol/min) (PHU/min) Ratio Ratio Stop Flow 10.23 0.15 0.01 1.53 0.23 Stop Flow 2 0.24 0.12 0.01 1.93 0.25 Stop Flow3 0.30 0.10 0.01 3.04 0.36 Stop Flow 4 0.27 0.11 0.01 2.45 0.38 StopFlow 5 0.40 0.20 0.02 1.99 0.26 Stop Flow 6 0.33 0.10 0.01 3.34 0.29

Average baseline values compared to treatment values illustrate that a2-minute exposure of CHO-M3 cells to carbachol resulted in a doubling inthe pH rate (0.01 PHU/min vs 0.02 PHU/min), though O₂ and CO₂ rates alsodemonstrated increases above baseline levels (see below). Post-treatmentrates generated during stop flow period number 6 essentially returned topre-treatment values.

Example 7 Comparison of Measured Analyte Fluxes with and without theTemporary Formation of a Sample Chamber

The temporary formation of a sample chamber within a larger vessel, andcontaining an effectively high concentration of cells, is acharacteristic for the instant invention.

In order to demonstrate this principle, a cylindrical vessel ofapproximately 12 mm in diameter and 10 mm in height was constructed frompolycarbonate material. Fluorescent sensors capable of measuring partialpressures of O₂ and CO₂, and a sensor capable of measuring pH, wereinstalled in the bottom of the vessel and calibrated as describedpreviously. A cylindrical cover was also fabricated from polycarbonatematerial, with a diameter to accommodate insertion into the vessel, inorder to provide a gas-impermeable cover and to reduce the enclosedvolume of the vessel as required. A cylindrical spacer of 0.5 mm inheight was also fabricated, with a diameter to accommodate insertion inthe bottom of the vessel, thus providing a stop for the cover at aprecise location.

Approximately 1×10⁵ C2C12 myoblast cells was prepared as describedpreviously. The cells were placed within the vessel along withapproximately 1 ml of cell growth media. The partial pressure of oxygenwithin the cell media was measured continuously using a calibratedfluorescent probe. The rate of change of pO₂ was then calculated fromthe difference between the pO₂ values at 12 and 16 minutes from thestart of the experiment.

A cylindrical cover was then placed on the surface of the cell media inorder to inhibit the influx of oxygen from the ambient air. This alsoreduced the volume of media exposed to the cells to approximately 130μl. Again, the pO₂ values at 12 and 16 minutes from the start of theexperiment were measured and recorded.

The cylindrical cover was then lowered within the vessel (to rest on thespacer) so as to reduce the volume of media exposed to the cells toapproximately 574 Again, the pO₂ values at 12 and 16 minutes from thestart of the experiment were measured and recorded.

The measured rates of change of pO₂ within the cell media for the threeconditions are shown in tabular form in Table 7 and in graphical form inFIG. 12.

TABLE 7 Oxygen depletion rates in a four minute interval, with andwithout the formation of an enclosed sample chamber (n = 3) Volume ΔpO2s.d. % CV Small (57 μL) 0.38 0.04 10% Medium (130 μL) 0.12 0.05 38% Open−0.04 0.10 n/a

This experiment demonstrates that the formation of an enclosed samplechamber, sealed from ambient air and containing a high density of cells,generates flux rates sufficient to provide a rapid measurement with ahigh signal-to-noise ratio.

It is apparent that many modifications and variations of this inventionas hereinabove set forth may be made without departing from the spiritand scope of the present invention and the above examples are notintended to in any way to limit the present invention but are merelyexemplary. The foregoing embodiments are therefore to be considered inall respects illustrative rather than limiting on the inventiondescribed herein.

1-81. (canceled)
 82. An apparatus for analysis of cells disposed inmedia in multiple wells of a multi-well plate, the apparatus comprising:a plurality of barriers for insertion into respective wells of themulti-well plate, each barrier comprising a barrier surface thatdefines, when inserted into the respective well, a sample chamber havinga reduced volume of medium less than 50% of the original volume ofmedium in the wells, and disposed on barrier surfaces, fluorescentsensors for analyzing a constituent of the medium disposed about thecells in the respective sample chamber.
 83. The apparatus of claim 82wherein the plurality of barriers comprises 24 barriers defining anarray.
 84. The apparatus of claim 82 wherein the plurality of barrierscomprises 96 barriers defining an array.
 85. The apparatus of claim 82wherein the fluorescent sensors sense an oxygen concentration in themedium.
 86. The apparatus of claim 82 wherein the fluorescent sensorssense a hydrogen ion concentration in the medium.
 87. The apparatus ofclaim 82 wherein the plurality of barriers defines a removable cover.88. The apparatus of claim 87 wherein the removable cover is disposable.89. The apparatus of claim 88 wherein each barrier has a distal end, andthe sensors are disposed on barrier surfaces of the respective distalends.